1. Field of the Invention
The present invention relates to a method of kneading ink of a printing press and a printing press.
2. Related Art
Among conventional printing presses, some printing presses are structured to have an ink fountain for storage of printing ink (hereinafter simply referred to ink), an ink fountain roller that rotates so as to make ink of the ink fountain adhere to its circumference and feed the ink therefrom to a downstream roller (e.g., an ink transfer roller), and one or more ink fountain keys that are aligned in a first direction parallel to the axis of the ink fountain roller and disposed so as to be moved back and forth in a second direction orthogonal to the first direction, thereby keeping a predetermined gap or space (hereinafter referred to gap) between the ink fountain roller and each of the ink fountain keys and hence determining the amount of ink to be fed to the downstream roller.
In the printing press of the above type, for example, ink fed from the ink fountain to the ink fountain roller is fed via a group of rollers comprising a number of rollers, which include ink transfer rollers disposed downstream to the ink fountain roller, to a plate cylinder with printing plates mounted thereon and then transferred from the plate cylinder to a rubber cylinder (a blanket cylinder) as a printing image. Ink which has been transferred as the printing image from the plate cylinder to the rubber cylinder is then transferred to substrates (e.g., printing sheets) that pass between the rubber cylinder and its corresponding impression cylinder, thereby printing the printing image on the substrates. Thus, the substrates with the printing image thereon (prints) are provided. In this printing operation, a predetermined gap is kept between each of the ink fountain keys and the ink fountain roller, thereby allowing a proper amount of ink for adjusting an image quality (image density) of prints to be fed from the ink fountain roller to the ink transfer roller. Thus, a desired image quality for prints can be attained. The image quality of prints to be printed is easy to be influenced by the characteristic change of ink. For example, ink in the ink fountain has its characteristics changed and viscosity generally increased in a case where the ink is left therein for a long time since the last printing operation of the printing press. It is known that the following phenomena may take place in such a case.
That is, in the printing operation, ink is kneaded by the rotation of the ink fountain roller as the printing proceeds and such kneading affects on ink, causing, in general, ink to have a viscosity lower than the viscosity of ink which has been left for a long time since the last printing operation. In more detail, in a case where the printing operation has been initiated with ink left in the ink fountain for a long time, the viscosity of this ink is increased at an initial stage of the printing operation, and therefore this highly viscous ink may likely cause print images to be printed on prints with a decreased ink density. As the printing operation proceeds further, the viscosity of ink is gradually decreased, thereby likely causing printing images to be printed on prints with an increased density. This change in density of ink printed on prints, due to the characteristic change (particularly, viscosity change) of ink between the initial stage and later stages of the operation, may make it easy to cause, for example, so-called broke or prints which cannot be used as products in the initial stage of the printing operation.
In a conventional printing press, prior to starting the printing operation, a given operational mode adapted such as for maintenance, trouble repair or more specifically cleaning of an ink fountain roller is selected so that the ink fountain roller is rotated for a given time (e.g., several ten minutes) with the ink fountain keys being held at a given distance from the ink fountain roller or, for example, being opened to the maximum so as to knead ink in the ink fountain by the rotation of the ink fountain roller, thereby allowing the viscosity of the ink to be the same level as the viscosity of ink attained in the later stages of the printing operation. Thus, the printing operation is started.
In order to further improve an ink kneading effect produced by the rotation of the ink fountain roller, additional ink kneading is sometimes made manually by an operator using a paddle-like tool or the like, or by an ink kneading unit that is provided independently of the printing press. This unit is equipped such as with a rotary stirrer having a circular-cone shape that is disposed to be movable in a direction parallel to the axis of the ink fountain roller relative to the ink fountain, in which the rotary stirrer is rotated in the ink fountain as it is moved in the direction parallel to the axis of the ink fountain roller.
The above conventional kneading operation, which is made additionally for the improvement of the kneading effect by the rotation of the ink fountain roller, increases the operator's workload to manually knead ink in the ink fountain, or wash the stirrer of the kneading unit after the kneading operation. Whether the kneading operation is made manually or automatically, the operator's workload is increased. As a further disadvantage in providing an ink kneading unit, the cost of the printing press is increased due to the additional arrangement of the ink kneading unit.
In consideration of the above problems, it is an object of the present invention to provide a method of kneading ink of a printing press and a printing press that are capable of reducing the operator's workload and holding down the cost of the printing press.